Anelectromagnetic field (alsoEM field) is aphysical field, varying in space and time, that represents the electric and magnetic influences generated by and acting uponelectric charges.[1] The field at any point in space and time can be regarded as a combination of anelectric field and amagnetic field. Because of the interrelationship between the fields, a disturbance in the electric field can create a disturbance in the magnetic field which in turn affects the electric field, leading to an oscillation that propagates through space, known as anelectromagnetic wave.[2][3]
Mathematically, the electromagnetic field is a pair ofvector fields consisting of one vector for the electric field and one for the magnetic field at each point in space. The vectors may change over time and space in accordance withMaxwell's equations. The vectors are subject to the rules ofspecial relativity; different observers may determine different vectors.
The way in which charges and currents (i.e. streams of charges) interact with the electromagnetic field is described byMaxwell's equations[4] and theLorentz force law.[5] Maxwell's equations detail how the electric field converges towards or diverges away from electric charges, how the magnetic field curls around electrical currents, and how changes in the electric and magnetic fields influence each other. The Lorentz force law states that a charge subject to an electric field feels a force along the direction of the field, and a charge moving through a magnetic field feels a force that is perpendicular both to the magnetic field and to its direction of motion.
Results of Michael Faraday's iron filings experiment.
The empirical investigation of electromagnetism is at least as old as the ancient Greek philosopher, mathematician and scientistThales of Miletus, who around 600 BCE described his experiments rubbing fur of animals on various materials such as amber creating static electricity.[7] By the 18th century, it was understood that objects can carry positive or negativeelectric charge, that two objects carrying charge of the same sign repel each other, that two objects carrying charges of opposite sign attract one another, and that the strength of this force falls off as the square of the distance between them.Michael Faraday visualized this in terms of the charges interacting via theelectric field. An electric field is produced when the charge is stationary with respect to an observer measuring the properties of the charge, and amagnetic field as well as an electric field are produced when the charge moves, creating an electric current with respect to this observer. Over time, it was realized that the electric and magnetic fields are better thought of as two parts of a greater whole—the electromagnetic field. In 1820,Hans Christian Ørsted showed that an electric current can deflect a nearby compass needle, establishing that electricity and magnetism are closely related phenomena.[8] Faraday then made the seminal observation that time-varying magnetic fields could induce electric currents in 1831.
In 1861,James Clerk Maxwell synthesized all the work to date on electrical and magnetic phenomena into a single mathematical theory, from which he then deduced thatlight is an electromagnetic wave. Maxwell's continuous field theory was very successful until evidence supporting the atomic model of matter emerged. Beginning in 1877,Hendrik Lorentz developed an atomic model of electromagnetism and in 1897J. J. Thomson completed experiments that defined theelectron. The Lorentz theory works for free charges in electromagnetic fields, but fails to predict the energy spectrum for bound charges in atoms and molecules. For that problem,quantum mechanics is needed, ultimately leading to the theory ofquantum electrodynamics.
Practical applications of the new understanding of electromagnetic fields emerged in the late 1800s. The electrical generator and motor were invented using only the empirical findings like Faraday's and Ampere's laws combined with practical experience.
There are different mathematical ways of representing the electromagnetic field. The first one views the electric and magnetic fields as three-dimensionalvector fields. These vector fields each have a value defined at every point of space and time and are thus often regarded as functions of the space and time coordinates. As such, they are often written asE(x,y,z,t) (electric field) andB(x,y,z,t) (magnetic field).
If only the electric field (E) is non-zero, and is constant in time, the field is said to be anelectrostatic field. Similarly, if only the magnetic field (B) is non-zero and is constant in time, the field is said to be amagnetostatic field. However, if either the electric or magnetic field has a time-dependence, then both fields must be considered together as a coupled electromagnetic field usingMaxwell's equations.[9]
With the advent ofspecial relativity, physical laws became amenable to the formalism oftensors. Maxwell's equations can be written in tensor form, generally viewed by physicists as a more elegant means of expressing physical laws.
The behavior of electric and magnetic fields, whether in cases of electrostatics, magnetostatics, orelectrodynamics (electromagnetic fields), is governed by Maxwell's equations. In the vector field formalism, these are:
where is the charge density, which is a function of time and position, is thevacuum permittivity, is thevacuum permeability, andJ is the current density vector, also a function of time and position. Inside a linear material, Maxwell's equations change by switching the permeability and permittivity of free space with the permeability and permittivity of the linear material in question. Inside other materials which possess more complex responses to electromagnetic fields, these terms are often represented by complex numbers, or tensors.
TheLorentz force law governs the interaction of the electromagnetic field with charged matter.
When a field travels across to different media, the behavior of the field changes according to the properties of the media.[10]
Electric field of a positive pointelectric charge suspended over an infinite sheet of conducting material. The field is depicted byelectric field lines, lines which follow the direction of the electric field in space.
The Maxwell equations simplify when the charge density at each point in space does not change over time and all electric currents likewise remain constant. All of the time derivatives vanish from the equations, leaving two expressions that involve the electric field,andalong with two formulae that involve the magnetic field:andThese expressions are the basic equations ofelectrostatics, which focuses on situations where electrical charges do not move, andmagnetostatics, the corresponding area of magnetic phenomena.[11]
Whether a physical effect is attributable to an electric field or to a magnetic field is dependent upon the observer, in a way thatspecial relativity makes mathematically precise. For example, suppose that a laboratory contains a long straight wire that carries an electrical current. In the frame of reference where the laboratory is at rest, the wire is motionless and electrically neutral: the current, composed of negatively charged electrons, moves against a background of positively charged ions, and the densities of positive and negative charges cancel each other out. A test charge near the wire would feel no electrical force from the wire. However, if the test charge is in motion parallel to the current, the situation changes. In the rest frame of the test charge, the positive and negative charges in the wire are moving at different speeds, and so the positive and negative charge distributions areLorentz-contracted by different amounts. Consequently, the wire has a nonzero net charge density, and the test charge must experience a nonzero electric field and thus a nonzero force. In the rest frame of the laboratory, there is no electric field to explain the test charge being pulled towards or pushed away from the wire. So, an observer in the laboratory rest frame concludes that amagnetic field must be present.[12][13]
In general, a situation that one observer describes using only an electric field will be described by an observer in a different inertial frame using a combination of electric and magnetic fields. Analogously, a phenomenon that one observer describes using only a magnetic field will be, in a relatively moving reference frame, described by a combination of fields. The rules for relating the fields required in different reference frames are theLorentz transformations of the fields.[14]
Thus, electrostatics and magnetostatics are now seen as studies of the static EM field when a particular frame has been selected to suppress the other type of field, and since an EM field with both electric and magnetic will appear in any other frame, these "simpler" effects are merely a consequence of different frames of measurement. The fact that the two field variations can be reproduced just by changing the motion of the observer is further evidence that there is only a single actual field involved which is simply being observed differently.
Reciprocal behavior of electric and magnetic fields
The two Maxwell equations, Faraday's Law and the Ampère–Maxwell Law, illustrate a very practical feature of the electromagnetic field. Faraday's Law may be stated roughly as "a changing magnetic field inside a loop creates an electric voltage around the loop". This is the principle behind theelectric generator.
Ampere's Law roughly states that "an electrical current around a loop creates a magnetic field through the loop". Thus, this law can be applied to generate a magnetic field and run anelectric motor.
Behavior of the fields in the absence of charges or currents
James Clerk Maxwell was the first to obtain this relationship by his completion of Maxwell's equations with the addition of adisplacement current term toAmpere's circuital law. This unified the physical understanding of electricity, magnetism, and light: visible light is but one portion of the full range of electromagnetic waves, theelectromagnetic spectrum.
A notable application of visible light is that this type of energy from the Sun powers all life on Earth that either makes or uses oxygen.
A changing electromagnetic field which is physically close to currents and charges (seenear and far field for a definition of "close") will have adipole characteristic that is dominated by either a changingelectric dipole, or a changingmagnetic dipole. This type of dipole field near sources is called an electromagneticnear-field.
Changingelectric dipole fields, as such, are used commercially as near-fields mainly as a source ofdielectric heating. Otherwise, they appear parasitically around conductors which absorb EMR, and around antennas which have the purpose of generating EMR at greater distances.
Changingmagnetic dipole fields (i.e., magnetic near-fields) are used commercially for many types ofmagnetic induction devices. These include motors and electrical transformers at low frequencies, and devices such asRFID tags,metal detectors, andMRI scanner coils at higher frequencies.
The potential effects of electromagnetic fields on human health vary widely depending on the frequency, intensity of the fields, and the length of the exposure. Low frequency, low intensity, and short duration exposure to electromagnetic radiation is generally considered safe.[17] On the other hand, radiation from other parts of theelectromagnetic spectrum, such asultraviolet light[18] andgamma rays,[19] are known to cause significant harm in some circumstances.
